1. Field of the Invention
The present invention relates to an electron beam adjusting device for use in cathode-ray tubes (CRTs) of television receivers, displays, etc. More particularly, the present invention relates to an electron beam adjusting device which is improved in the accuracy of adjustment made thereby, reduced in size and thickness, and also improved in the degree of freedom with which it is attached around the neck of a CRT.
2. Description of the Prior Art
Referring first to FIGS. 6, and 7, a typical conventional electron beam adjusting device comprises a magnet assembly formed by combining together ring magnets 1a, 1a', 1b, 1b', 1c and 1c' and spacers 2a, 2b and 2c, a cylindrical holder 3 made of a resin material, to which the magnet assembly is attached, and a metallic band screw 4 and a lock ring 5, which are used to secure the magnet assembly to the neck of a cathode-ray tube (CRT). It has heretofore been common practice to use as the ring magnets bonded magnets (resin-bonded magnets) each having a thickness of 1.2 to 1.6 mm and as the spacers those which are made of a resin material and have a thickess of 1 to 3 mm and a part of which has a spring mechanism. In the commonest type of electron beam adjusting device, the thickness d1 of the magnet assembly comprising the magnets and the spacers is 15 mm, and the overall thickness d2 of the adjusting device, including the holder 3, is 25 mm or more, although it somewhat varies according to the basic structure. It is common for the type of electron beam adjusting device with a lock mechanism to have a larger thickness than the above.
The ring magnets include two-, four- and six-pole magnets each having a thickness of 1.2 to 1.6 mm, which are formed by injection molding as moldings with the same thickness and thereafter magnetized such that the inner peripheral surfaces of the moldings have two, four and six magnetic poles, respectively. The magnet assembly comprises pairs of two-, four- and six-pole magnets 1a, 1a', 1b, 1b', 1c and 1c', which are attached to a predetermined holder 3 with spacers 2a, 2b and 2c interposed between the pairs of magnets, respectively, thereby forming an electron beam adjusting device. The functions of the pairs of magnets in the electron beam adjusting device are as follows: The pair of two-pole magnets converges each of the three beams to the center of the fluorescent screen; the pair of four-pole magnets superposes two side beams one upon the other; and the pair of six-pole magnets superposes the two side beams superposed by the four-pole magnets upon the center beam. Each pair of magnets is used in such a manner that, after the magnets are rotated in opposite directions to adjust the magnetic force to a necessary level, the two magnets are rotated together in the same direction for adjustment. The intensity of magnetic force of each pair of magnets determines the amount of shift of the beam, that is, the maximum adjusting width. The minimum amount of beam shift, that is, the adjusting accuracy, is determined by the uniformity of magnetic fields in the two-pole magnets and by magnetic force variations between the poles of each magnet in the four- and six-pole magnets. In electron beam adjusting devices used in high-precision CRTs for displays or the like, it is particularly essential to reduce interpole magnetic force variations in the four- and six-pole magnets to thereby reduce the minimum amount of beam shift and thus improve the adjusting accuracy.
Incidentally, in order to ensure the required magnetic force intensity and uniformity of magnetic fields in two-pole magnets, which are the most difficult to ensure the magnetic force intensity, the thickness of the two-pole magnets must be increased to a certain extent. However, in the prior art wherein all the two-, four- and six-pole magnets have the same thickness, if the thickness of the two-pole magnets is increased, the thicknesses of the four-and six-pole magnets are also increased, as a matter of course. Since the four- and six-pole magnets need not so strong magnetic force as the two-pole magnets do but can exhibit the required functions with a relatively low level of magnetic force, when all the magnets have the same thickness, the amount of magnetization of the four- and six-pole magnets needs to be held down below that for the two-pole magnets. However, it is difficult to control the amount of magnetization to a relatively low level because bonded magnets, which are molded out of a magnetic material, unavoidably involve variations in the magnetic properties of the material, variations in the dimension and density of the moldings, variations in the magnetizing voltages, etc. This difficulty in contol causes variations in the amount of magnetization between the poles. Particularly, when there are large variations in the initial magnetization characteristics due to the powdered magnetic material and a large play between the inner peripheries of the moldings and the magnetizing yoke, it is practically difficult to stabilize the amount of magnetization at a relatively low level while adjusting these variations with the magnetizing voltage.
In addition, when the magnets are thick, it is also necessary to minimize aberration by providing a magnetic force difference between magnets which pair with each other, depending upon the CRT structure, so that the magnetization adjustment is complicated, which causes an increase in the interpole magnetic force variations.
Further, considering the attachment of an electron beam adjusting device to the neck of a CRT, together with adjusting characteristics and costs, it is extremely important to ensure the degree of freedom with which the electron beam adjusting device is mounted in position and to reduce the weight. Accordingly, the magnets become more favorable as the thickness and weight thereof decrease, as long as it is possible to ensure the strength required during adjustment, that is, the breaking strength of a portion of each magnet which is pinched to make adjustment. However, in the case of magnet moldings which have heretofore been used, when the thickness of the moldings is 1.0 mm or less, the flame retardance of the composition and the strength of the magnet moldings lower markedly. For this reason, the thickness of the moldings cannot be reduced to 1.0 mm or less in the present state of art.